Air bags are inflatable bags that remain folded and out of sight in readiness for a frontal collision. During a collision, chemical reaction of a material typically containing sodium azide produces gaseous products which inflate the bag and interpose it between the driver and the steering wheel or the front seat occupant and the dashboard.
Belt tensioners are devices that tighten automobile seat belts during a crash to hold the seat occupants more securely. A tensioner has a small motor driven by pressurized gas from the chemical reaction of gas generating material like that used for inflating air bags. In both cases, chemical reaction of the gas generating material is initiated by an electrically heated squib. The crash sensor of this invention is a switch for controlling the power that energizes the squib.
In the United States automobiles having electrically initiated air bag inflators are required by law to incorporate diagnostic capability to warn the driver of a failure in the firing circuit.
This invention is an improvement on a crash sensor which is in commercial production. The crash sensor in current production consists of a ball free to move in a sealed tube with contacts that are bridged by the ball when it moves beyond a predetermined distance. Viscous air flow around the ball causes a pressure differential which resists ball movement in proportion to the velocity of the ball with respect to the tube. The proportionality of viscous force to velocity makes the crash sensor an acceleration integrator that completes the firing circuit when a crash causes a predetermined vehicular velocity change. A permanent magnet retains the ball in a normal resting position away from the contacts and causes the vehicular velocity change required for switch closure to increase with the duration of the crash. To compensate for the variation of air viscosity with temperature the tube and ball are made of different stainless steels having a difference in their thermal expansion coefficients such that the gap between the ball and tube changes with temperature to maintain constant performance of the crash sensor over a wide temperature range. An elastomeric seal reduces transmission of cross axis vibrations to the ball and tube.
This known sensor is expensive to manufacture and one reason for the high cost is the high precision required of the ball and the tube. Another reason for the high cost is that the tube material is difficult to work. Another reason for the high cost is the requirement in the present design that the electrical contacts must remain in good electrical contact with the ball as the ball moves over a distance of about one fourth of an inch. Further, the contacts are connected with lead wires and a diagnostic resistor by soldering. This creates contamination during manufacturing that cannot be tolerated in the vicinity of the ball and tube thereby requiring additional components and processing steps to maintain isolation for cleanliness. This known sensor is particularly affected by contamination because small particles can wedge between the ball and the tube and interfere with movement of the ball.
The performance of this current production crash sensor depends upon flow of air between the ball and the inner diameter of the tube which varies depending on whether the ball moves near the center of the tube or near the wall. During a crash the venturi effect creates aerodynamic forces that urge the ball toward the center of the tube whereas lateral accelerations urge the ball toward the wall. When the ball moves near the wall there is a crescent shaped air duct between the ball and the tube. When the ball moves near the center of the tube there is an annular shaped air duct between the ball and the tube. The crescent shaped opening has about one half of the resistance to air flow as the annular opening. Therefore, the velocity change required for the ball to bridge the contacts can vary substantially depending on the path of the ball.
When the velocity of the air between the ball and tube of this current production crash sensor is large the flow rate is substantially affected by the need for the pressure differential across the gap to overcome the inertia of the air and accelerate it to the maximum velocity it achieves at the narrowest point between the ball and tube. The pressure required to overcome inertia reduces the pressure available to overcome viscous resistance to flow. Furthermore, the pressure required to overcome inertia is independent of temperature because it does not depend on viscosity and causes the temperature compensation resulting from the aforemenioned different expansion coefficients to be excessive. Also, inertial damping of air flow does not vary linearly with pressure so the sensor is not a velocity integrator under these circumstances.
A safing sensor is a second crash sensor typically located on the firewall or elsewhere in the interior of the vehicle and is wired in series with one or more crash sensors located near the front of the vehicle. It prevents deployment of occupant protection systems in the event of sharp blows to a forward crash sensor which would not warrant deployment of the system.
Diagnostic systems typically include a diagnostic resistor that supplies a small current to the firing circuit during normal operation and means for monitoring that small current. Absence of the current indicates an open firing circuit. It is also desirable for the diagnostic system to detect small variations in the resistance of the firing circuit such as might be caused by corrosion of connectors. Circuits to accomplish this require high precision and current systems do not perform this function as well as is desired.
A switching circuit for measuring resistance is described in U.S. Pat. No. 5,115,188 issued May 19, 1992 to Peter Norton for Resistance Sensor And Switch.
U.S. Pat. No. 4,932,260 issued Jun. 12, 1990 to Peter Norton for "Crash Sensing Switch With Suspended Mass" describes a crash sensor having a suspended mass in which air ducts conduct the air displaced by the movement of the sensing mass and in which compensation for variation of air viscosity with temperature is accomplished by varying the normal resting position of the sensing mass according to the temperature thereby by requiring the sensing mass to travel farther at lower temperatures before bridging the contacts and less far at higher temperatures.
Copending application Ser. No. 249,840 filed May 26, 1994 and titled "Crash Sensing Switch" describes a crash sensor having a cylindrical sensing mass movable in a tube in which air ducts incorporated in the sensing mass conduct the air displaced by the sensing mass. The ball shaped sensing mass of the invention of the present application could be replaced by a cylindrical sensing mass having air ducts and means for reducing the friction between the sensing mass and tube. This provides the advantages of small sensing mass and air ducts without the need to form those ducts in the wall of the tube or provide for ducts outside the inside diameter of the tube.
Integrated electronic circuits commonly include on a single chip of silicon both switching power transistors for controlling substantial power and circuitry for making the decisions as to when the switching power transistors should be conducting and not conducting. This circuitry may include voltage comparators, timing devices, and circuits for performing boolean logic.
Connections to semiconductor devices including integrated circuits are commonly made by attaching fine wires of materials such as gold or aluminum to pads on the semiconductor formed for that purpose. The pads are areas of the semiconductor chip that are sufficiently large for attachment to be made reliably by methods such as acoustic welding. The pads and the connections between the pads and the circuits on the semiconductor chip are made as part of the process of making the circuitry on the semiconductor chip.
Completed semiconductor chips are commonly mounted on a substrate which may be metal, ceramic, plastic, or other suitable material. The substrate typically includes connector pins for connecting the completed semiconductor device to other equipment. The pads on the chip are commonly connected to the connector pins by stitch bonding. The delicate chip and wiring of the electrically complete device are protected by encapsulation. Many encapsulation methods are known and several have been highly developed to meet the needs of the semiconductor industry. One method is to cover the completed chip and wiring with a small can and solder the can to the substrate which forms a hermetic seal. Another is to place the completed chip and wiring in a mold and flood the mold with uncured thermosetting plastic which is cured to form the required encapsulation.
Circuits for driving MOSFET power transistors are described in the literature. A review of such circuits is provided by chapter 6 of the second edition of POWER MOSFET TRANSISTOR DATA copyright 1986 and published by Motorola Inc.
A general object of this invention is to provide a crash sensor and diagnostic system for automotive vehicles which overcomes certain disadvantages of the prior art.